Nanodevices which are highly thermally responsive to the excitation in the near infrared spectral region (NIR) are a powerful tool in photoablation of cancer cells because of the efficient penetration of this radiation throughout the tissues, without damaging the healthy cells surrounding the tumour. Two- or multi-photon excitations by pulsed NIR laser can trigger high-energy transitions normally requiring UV-Vis excitations. Engineered gold nanoparticles (AuNPs), which are shape modelled to shift absorptions in the NIR region, are cheap, non-toxic and efficient therapeutics for applications in this field. Spherical AuNPs typically show low absorption in the NIR; when properly functionalized with glutathione corona carrying the dansyl chromophore (DG-AuNPs) they spontaneously produce aggregates with enhanced response to the pulsed excitation in the NIR region where the chromophore acts as antenna. The energy harvested by the dansyl functionality is transferred to the AuNPs where it is quenched via the neighbour nanoparticle effect in the aggregates; thus, the NIR excitation is completely converted into heat, as a consequence. Surprisingly the aggregates of DG-AuNPs are readily internalized in human liver cancer cells (line Hep G2) and mouse neuronal cells, via invagination of vesicles (see Figure); noteworthy they do not affect the cellular vitality also in long term treatment. Hep G2 cells incubated for 1 h with a suspension of DG-AuNPs (0.07 mg/mL) and irradiated soon later for 1.26s with pulsed laser tuned at 760 nm are efficiently photoablated. Moreover the DG-AuNPs show, after their internalization, a residual fluorescence in the UV-Vis region that is attributed to the single AuNPs produced from disaggregation under biological environment in the cell (indicated by arrows in Figure); this behaviour allows preserving their application in cellular imaging.
In summary, aggregates of gold nanoparticles functionalized with dansyl-glutathione, synthesised using a simple and fast methodology, are both efficient absorbers and quenchers of pulsed NIR suggesting their potential use in theranostics.